Source: https://balanceandtest.com/resources/balancing-machine-acceptance-tests/ - **Acceptance Tests: URR and Umar** Ask an Engineer - [Home](https://balanceandtest.com/) - [Resources](https://balanceandtest.com/resources/) - Acceptance Tests: URR and Umar Balancing guide # Balancing Machine Acceptance Tests What unbalance reduction ratio (URR) and minimum achievable residual unbalance (Umar) mean, how ISO 21940-21 tests them, and how to read a balancing machine spec. Read the Article Ask an Engineer Contents - Why a balancing machine needs a test of its own - Minimum achievable residual unbalance (Umar) - Unbalance reduction ratio (URR) - How the tests are run - Acceptance at the factory and on site - Reading a balancing machine quotation - Keeping the machine in specification Written by [Steve Fowler](https://balanceandtest.com/about-us/steve-fowler/) , Managing Director, Balance & Test. 35+ years in rotor balancing; project manager for the rewrite of ISO 21940-21. **Key facts:** Two figures describe how good a balancing machine is. **Umar** , the minimum achievable residual unbalance, is the smallest unbalance the machine can get a rotor down to. **URR** , the unbalance reduction ratio, is the share of the unbalance it removes in a single measure-and-correct step. ISO 21940-21 defines how both are tested, using a proving rotor and known test masses. ## Why a balancing machine needs a test of its own A balancing machine tells you a rotor is balanced by showing a small reading. That only means something if the reading is right. A machine with a fault can show zero on a rotor that is out, or send the operator chasing an unbalance that is in the tooling. An acceptance test takes the rotor out of the question. A **proving rotor** of known geometry is first balanced as finely as the machine allows. **Test masses** of known size are then fitted at known positions, and the machine has to report what was fitted. Because the answer is known in advance, any difference is the machine's error. ## Minimum achievable residual unbalance (Umar) Umar is the floor: the smallest residual unbalance the machine can reliably achieve. Below it, the machine cannot tell real unbalance from its own noise, friction and drive effects. It is usually quoted as a specific unbalance, in g·mm per kg of rotor mass. That unit is the same as micrometers: it is how far the rotor's center of mass sits from its axis. Quoting it this way lets one figure cover rotors of different weights. A machine rated at 0.5 g·mm/kg can bring a 10 kg rotor to about 5 g·mm and a 100 kg rotor to about 50 g·mm. ### Is the machine good enough for the job? Compare Umar with the tolerance you have to meet, expressed the same way. From the balance quality grade and the service speed: eper = 9,549 × G ÷ n (in g·mm/kg, with n in rpm) | Rotor | Tolerance eper | Margin on a 0.5 g·mm/kg machine | | G 6.3 at 1,500 rpm | 40 g·mm/kg | 80 times | | G 2.5 at 3,000 rpm | 8 g·mm/kg | 16 times | | G 2.5 at 16,000 rpm | 1.5 g·mm/kg | 3 times | The first two are comfortable. The third is not: a machine working that close to its own limit will pass and fail the same rotor on different runs. As a working rule the machine should be several times better than the tightest tolerance it will be asked to hold, and high-speed rotors are where that rule decides which machine you need. ## Unbalance reduction ratio (URR) URR measures accuracy in a different way: not how low the machine can go, but how well it tells you what to do. It is the proportion of the initial unbalance removed by one correction made exactly as the machine indicated: URR = (initial unbalance − remaining unbalance) ÷ initial unbalance, as a percentage A rotor arriving with 200 g·mm, corrected once: | URR | Left after one correction | Left after two | | 95% | 10 g·mm | 0.5 g·mm | | 90% | 20 g·mm | 2 g·mm | | 80% | 40 g·mm | 8 g·mm | With a tolerance of 15 g·mm, the 95% machine finishes in one correction; the 90% and 80% machines need two. On a production line every extra step is another spin-up, another cut and another spin-down. URR is therefore the figure that sets cycle time, and it matters most on automatic machines, where the correction is made without an operator's judgment. URR depends on the machine reading both the **amount** and the **angle** correctly, and on keeping the two correction planes apart. An error of a few degrees in angle leaves a residual however accurate the amount. URR is the share of the measured unbalance that one correction run removes. Balance & Test machines are specified at 95%. ## How the tests are run ISO 21940-21 describes the procedures. In outline: - **Proving rotors.** The standard defines proving rotors of set proportions, with rings of tapped holes for test masses. There are different types for disc rotors on vertical machines, rotors supported between bearings and overhung rotors, in a range of sizes to suit the machine's capacity. - **The Umar test.** The proving rotor is balanced to the claimed limit. A small test mass is then fitted at a series of angular positions in turn and the machine's readings recorded. If the machine really works at the claimed level, the readings follow the test mass round in amount and angle within the limits the standard sets. - **The URR test.** Larger test masses are used, one left in a fixed position and one moved through a series of positions, so that the machine sees many combinations of amount and angle in both planes. The readings are compared with the known values, and they must fall inside the limits for the URR claimed. - **Other checks.** The standard also covers how far couple unbalance disturbs a single-plane reading, and how well the machine's compensation for tooling error works. The results are recorded on a test sheet and form the basis of the machine's certificate. ## Acceptance at the factory and on site A new machine is normally tested twice. - **Factory acceptance** is carried out before shipment, at the maker's works, with the buyer present if they wish. Things worth witnessing: the proving-rotor tests above; a run with your own part and tooling; changeover between part types; the guarding and safety functions; and, on an automatic machine, a batch of parts corrected without intervention. - **Site acceptance** repeats the key tests after installation. A balancing machine is sensitive to what it stands on: foundations, floor vibration from nearby plant, and the air and power supplies can all change the result. On our machines we write acceptance in two parts: your own rotors are balanced to **your** tolerance, and the machine's own figure is proved separately on a proving rotor to ISO 21940-21. That way each side is tested against something it controls. For a production machine, buyers usually add a repeatability and capability study with production parts: the same parts measured repeatedly, by more than one operator where loading is manual, to show the spread is a small fraction of the tolerance. Gauge R&R or a gauge capability index (Cgk 1.67 is a common target) are the usual methods. This tests the machine together with its tooling and loading, which the proving-rotor tests don't. ## Reading a balancing machine quotation Five figures decide whether a machine fits the work: - **Capacity:** maximum rotor mass and diameter, and for horizontal machines the range of journal diameters and the distance between supports. Check the smallest rotor as well as the largest. - **Umar:** compare it with your tightest tolerance, as above. - **URR:** for automatic and high-volume work, ask what is guaranteed, not what is typical. - **Speed range:** enough to measure your rotor stably, and no more than its tooling and guarding allow. - **The test standard:** the figures should be stated as tested to ISO 21940-21. Figures without a test method cannot be compared between suppliers. Then ask what is included. On our quotes the machine is proved at factory acceptance before it ships, a proving rotor is available as an option, and our engineers commission, certify and train on site. Our spec tables list 0.1 to 1 g·mm/kg depending on machine size. In practice the small belt-drive machines do considerably better: an H10B typically reaches about 0.01 g·mm/kg. ## Keeping the machine in specification The acceptance figures describe the machine on the day it was tested. Wear in rollers and bearings, sensor and cable faults, and changes to the floor it stands on all move them. Repeating the tests at intervals, and keeping the certificates, is how you show the machine still meets them. See [the balancing machine calibration guide](https://balanceandtest.com/resources/balancing-machine-calibration-guide/) . Machines and service ## Balancing Machines and Calibration Service Tested. Then kept in spec. [Service ### Balancing machine calibration service View the service ](https://balanceandtest.com/services/balancing-machine-calibration/) [Machines ### Horizontal balancing machines View machines ](https://balanceandtest.com/balancing-machines/horizontal-balancers/) [Machines ### Vertical balancing machines View machines ](https://balanceandtest.com/balancing-machines/vertical-balancers/) [Machines ### Balancing machines View all machines ](https://balanceandtest.com/balancing-machines/) Further reading ## Related Balancing Articles Keep reading. [Article ### ISO 21940 balance grades explained Read the article ](https://balanceandtest.com/resources/iso-21940-balance-grades/) [Article ### The balancing machine calibration guide Read the article ](https://balanceandtest.com/resources/balancing-machine-calibration-guide/) [Article ### How to choose a balancing machine Read the article ](https://balanceandtest.com/resources/how-to-choose-a-balancing-machine/) Comparing machines? ## Talk to a Balancing Engineer Tell us about the rotor and the tightest tolerance you need to hold, or the machine you need tested, and we will advise. UK +44 117 405 4011 USA +1 616 377 1077 [Email info@balanceandtest.com](mailto:info@balanceandtest.com) Ask an Engineer